A display apparatus in which an image formed by a DMD (Digital Micromirror Device, registered trade mark), one of reflective light modulation devices, is projected and displayed on a screen by a projection optical system is known as described, for example, in Japanese Unexamined Patent Publication No. 2000-206452. The DMD includes a multitude of micromirrors arranged in a matrix, in which each micromirror takes two tilted states of ON and OFF states. In a projector apparatus that employs a DMD, the projection optical system is disposed such that, when illumination light is directed to the DMD, illumination light incident on an ON state micromirror is reflected toward the inside of the projection optical system while illumination light incident on an OFF state micromirror is reflected toward the outside of the projection optical system. Consequently, only the light reflected by ON state micromirrors is projected on the screen by the projection optical system, whereby a light and dark pattern display image is formed on the screen. In this case, one micromirror represents one pixel of the display image.
An overview of a DMD will now be described with reference to FIGS. 9 and 10. FIG. 9 is a plan view of a DMD, illustrating a drive structure of the DMD, and FIG. 10 is a sectional elevation of the DMD, illustrating a cross-sectional shape of a portion along the line A-A in FIG. 9. Note that these drawings illustrate one of a multitude of micromirrors in an enlarged form, and an actual DMD includes a multitude of micromirrors arranged in a matrix.
As illustrated in the drawings, the micromirror 2 of the DMD 1 takes two tilted states of ON state in which the micromirror is tilted, for example, by +12° and OFF state in which it is tiled by −12° by turning around a turning axis R. The turning axis R is set in a direction that forms an angle of 45° with a long or short side of the DMD 1 and illumination light L0 is directed to the surface of the DMD 1 from a direction orthogonal to the turning axis at an incident angle of 24° with respect to the surface of the DMD 1. The illumination light L0 directed to the DMD 1 becomes a projection light L1 having a reflection angle of 0° with respect to the surface of the DMD 1 when reflected by an ON state micromirror 2, while it becomes a projection light L2 having a reflection angle of −48° with respect to the surface of the DMD 1 when reflected by an OFF state micromirror 2. As only the projection light L1 with a reflection angle of 0° reflected by ON state micromirrors 2 enters the projection optical system, an image formed by controlling the light and dark of each micromirror 2 is projected on the screen. That is, in this case, one micromirror 2 of the DMD 1 represents one pixel of the image.
The term “surface of the DMD 1” as used herein is defined as a surface parallel to the surface that includes turning axes R of all micromirrors and traverses the surfaces of all the micromirrors 2.
In the mean time, in a projector apparatus that employs the DMD 1 as described above, it is necessary to configure the illumination optical system to satisfy two conditions: directing the illumination light L0 from a direction orthogonal to the turning axis R of the micromirror 2, i.e., from a direction that forms an angle of 45° with a long or short side of the DMD 1 and directing the illumination light L0 to the surface of the DMD 1 at an incident angle of 24° due to structural reasons of the DMD 1.
Consequently, in conventional projector apparatuses, an illumination optical system that uses a TIR prism (Total Internal Reflection Prism) to guide illumination light to the DMD 1, as illustrated in FIGS. 11 and 12, is frequently used. That is, the light emitted from the light source 3 is collected by the rod integrator 5 through the color wheel 4 and guided by the first mirror 6 and the second mirror 7 to the TIR prism 8 where the light is totally reflected, thereby directing the light from a predetermined direction (direction orthogonal to the turning axis of micromirror) to the DMD 1 at a predetermined incident angle (24° with respect to the surface of the DMD 1). Then, the illumination light directed in the manner described above is reflected by DMD 1 to perform light modulation and light transmitted through the TIR prism 8 after the light modulation is projected on a screen by the projection optical system 9.
An example TIR prism 8 used in a conventional projector apparatus will now be described in detail with reference to FIGS. 6, 7, and 8 which illustrate the perspective shape, lateral shape, and bottom shape respectively. Note that, in FIGS. 6 to 8, a long side direction and a short side direction of the DMD 1 are designated as X direction and Y direction respectively and a direction orthogonal to X and Y directions is designated as Z direction. As illustrated in FIGS. 6 to 8, the TIR prism 8 includes a first prism 8A and a second prism 8B, and totally reflects illumination light Lin guided from an optical system (not shown) at a total reflection surface P to direct totally reflected illumination light Lout from a predetermined direction and at a predetermined angle with respect to the DMD 1. The TIR prism 8 also transmits reflection light Lref from the DMD 1 through the total reflection surface P of the first prism 8A and outputs from the second prism 8B, thereby guiding the light Lref to the projection optical system.
For that purpose, in the TIR prism 8, an inclination angle α of the total reflection surface P is set so as to totally reflect the incident illumination light Lin and transmit the reflection light Lref from the DMD 1, and the direction of the total reflection surface P is set such that a projection line, which is a normal N to the total reflection surface projected on a surface, including the surface of the DMD 1, forms an angle of 90° with the micromirror turning axis R of the DMD 1 (FIGS. 7 and 8). Then, the illumination light is directed to the TIR prism 8 such that a projection line which is a travelling direction of the incident illumination light Lin and exit (reflection) illumination light Lout with respect to the total reflection surface P projected on a surface, including the surface of the DMD 1, forms an angle of 90° with the micromirror turning axis R of the DMD 1, as in the normal N to the total reflection surface.
In a conventional projector apparatus having the aforementioned DMD and TIR prism, however, there has been no choice but to use a thick prism as the TIR prism, as clearly indicated in FIG. 8, thereby causing a problem that the projector apparatus becomes thick as the consequence.
In view of the circumstances described above, the present inventor has already proposed a projector apparatus that may use a thin TIR prism in Japanese Unexamined Patent Publication No. 2002-350775. This projector apparatus is a projector apparatus in which light from a light source is guided to a TIR prism, light totally reflected by the TIR prism is reflected by a DMD to perform light modulation, and light transmitted through the TIR prism after the light modulation is projected on a screen by a projection optical system, wherein: the TIR prism has a total reflection surface for totally reflecting and guiding illumination light to the DMD and transmitting light modulated by the DMD, and is disposed such that a projection line, which is a normal vector to the total reflection surface viewed from a direction perpendicular to the surface of the DMD, forms an angle of less than 45° with a long or short side of the DMD, i.e., forms an angle of less than 90° with a micromirror turning axis of the DMD; and the illumination optical system causes the illumination light to be incident on the total reflection surface such that a projection line, which is an optical axis of the illumination light exits from the TIR prism to the DMD viewed from a direction perpendicular to the surface of the DMD, forms an angle of 45° with a long or short side of the DMD.